UT Invitational Programming Contest Fall 2019

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2019-11-23 19:00 UTC

## UT Invitational Programming Contest Fall 2019

#### End

2019-11-24 00:00 UTC
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# Problem HMartian Patrol

Being so close to a nuclear superpower, Martians take their defense very seriously. The Martian military base is structured as a binary tree rooted at $1$, with nodes numbered $1 \dots N$. Some Martian soldiers patrol paths on the tree, moving from node to node.

The Martian patrol system is very complex. Every Martian patrol soldier is assigned two nodes as endpoints for their patrol and moves back and forth between these endpoints, always taking the shortest path. No two soldiers share an endpoint, and a soldier’s left and right endpoints cannot be the same.

The patrol schedule works as follows. At the beginning of the day, all Martian soldiers start at their left endpoints. Each soldier starts moving from their left to their right endpoint sometime in the next $N - 1$ seconds, traversing one edge every second. This time is randomly chosen for each soldier independently by the Martian’s sophisticated algorithm to prevent patrol timings becoming predictable. While there is an element of randomness, the start time for each soldier is always chosen such that the soldier will reach their right endpoint before (or as soon as) the $N - 1$ seconds ends. Once a soldier starts moving, they keep moving until they reach their right endpoint. This $N - 1$ seconds is considered a patrol cycle. This procedure then repeats, only this time soldiers start from their right endpoint and move to their left, and so on.

(For any given node $n$, all nodes in its left subtree are considered “to the left”, and similarly all nodes in its right subtree are considered “to the right”. A soldier at node $i$ is “on the left” of a soldier at node $j$ if and only if $i$ is in $j$’s left subtree, or $i$ is in the left subtree of node $i$ and node $j$’s lowest common ancestor and $j$ is in its right subtree. A soldier being “ on the right” of another soldier is defined similarly.)

The soldiers get lonely patrolling the Martian base all day, so they like to gossip with their buddies. If it is possible for a soldier to meet (be at the same node or edge as) another soldier during their patrol given some start time assignments, then the soldiers consider each other buddies. Soldiers become sad if any of their buddies is always on one side during a patrol cycle (always on the left or at the same node, or always on the right or at the same node), as when they gossip it hurts their ear.

The Martians are also planning to add a super secret room to the military base. This room will be added as a child of some node in the base without violating the binary nature of the tree. Once this room is added, Martian soldiers will be allowed to have it as one of their patrol endpoints.

Your job is to count the number of ways to add the super secret room and assign patrol endpoints to any number of soldiers such that the assignment of endpoints is “non-sad”. An arrangement is considered “non-sad” if it is impossible for any soldier to be sad, regardless of which starting times the algorithm picks. Two arrangments are considered different if and only if there exists a node in one arrangement not in the other, or a soldier’s path in one arrangement doesn’t exist in the other. Note that soldiers themselves are not considered distinct. The answer may be large, so output it modulo $10^9 + 7$.

## Input

The first line contains an integer $N$ ($2 \leq N \leq 10^6$). $N$ lines follow, with the $i$th ($1$-indexed) line having two integers $l_ i, r_ i$, denoting the left and right child of node $i$. A $0$ denotes that there is no child.

## Output

A single integer, the number of non-sad arrangements of soldiers and rooms $\mod 10^9 + 7$.

Sample Input 1 Sample Output 1
3
2 3
0 0
0 0
36
Sample Input 2 Sample Output 2
10
2 5
3 0
4 0
0 0
6 10
7 8
0 0
9 0
0 0
0 0
63778